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Abstract

We demonstrate a low-loss coupling scheme between a silicon photonic waveguide and a hybrid-plasmonic waveguide. Measured coupling efficiencies reach up to 94% or −0.27 dB. The metal-insulator-semiconductor structure is fabrication-tolerant and adaptable to a wide range of materials including those used in CMOS processes. The coupler is a promising building block for low-loss active plasmonic devices.

Figures (4)

Fig. 1 (a) Cross-sectional schematic of the coupler showing the layer stacks of the photonic waveguide (left) and hybrid waveguide (right). (b) Symmetric supermode and (c) anti-symmetric supermode Ey field profiles of the coupler. (d) Top-view intensity profile of the coupler showing mode beating between the two waveguides along the propagation. The profile is a composite of two y cross-sections: the midpoint of the silicon for the left half (photonic) and the midpoint of the spacer layer for the right half (hybrid-plasmonic). In each of (b), (c), and (d), the fields are normalized to the maximum electric field intensity (i.e. |E|2 = 1).

Fig. 2 (a) Effective indices of the fundamental TM modes in the hybrid (solid green) and photonic (dashed green) waveguides as a function of the waveguide width. The grey dots mark the widths used for the measurements in Fig. 4. The propagation loss of the hybrid plasmonic mode is plotted in blue. (b) Dependence of the symmetric (solid green) and anti-symmetric (dashed green) supermode effective indices on the distance separating the two waveguides. The difference between the curves determines the coupling length (blue) as per Eq. (1). (c) Simulated spectral response of the coupler.

Fig. 3 (a) Top-view schematic of the devices used to measure the efficiency of the hybrid couplers. (b) Colourized SEM image of the coupling region in a fabricated device showing the hybrid waveguide (left) and photonic waveguide (right).

Fig. 4 (a) Hybrid plasmonic waveguide cutback measurements at the cross-port (green) and through port (blue) at a free-space wavelength λ = 1550 nm. The slope of the cross-port’s line of best fit gives the hybrid plasmonic propagation loss while the y-intercept gives the loss of two hybrid couplers. (b) Measured coupling efficiencies |κ|2 (green) and uncoupled powers |τ|2 (blue) for various designed gap widths. The solid lines are values calculated from simulations.